The present invention generally relates to a driving method for an ink jet recording head enabled to eject an ink droplet, and to an ink jet recording apparatus for recording images and characters on recording paper by using such an ink jet recording head. More particularly, the invention relates to a driving method for an ink jet recording head adapted to eject an extremely small amount of ink in an ink droplet, which can form a microdot, and to an ink jet recording apparatus for recording images and characters on recording paper by using such an ink jet recording head.
Some printers and plotters are well known as typical ink jet recording apparatus (hereunder referred to simply as recording apparatus). In these recording apparatus, the diameter of a dot recorded on recording paper, that is, the resolution of the apparatus is determined according to the quantity of ink of ink droplets ejected from an ink jet recording head. Therefore, the ink quantity of the ejected ink droplets is important. Thus, there has been proposed a recording apparatus having a recording head adapted to eject ink droplets, the respective ones of which have different amounts of ink, from the same nozzle orifice. This recording head has a pressure generating element adapted to cause variation in pressure in ink contained in a pressure chamber. The recording head is caused to eject ink droplets, the respective ones of which have different amounts of ink, by supplying a plurality of kinds of drive pulses, which differ in electric potential change pattern from one another, to the pressure generating element.
Further, a related microdot drive pulse, that is, a drive pulse for ejecting an extremely small amount of ink of an ink droplet has a decompressing component for largely decompressing the pressure chamber, a decompressed state holding component for holding a decompressed state of the pressure chamber, and a compressing component for compressing the pressure chamber so as to eject ink droplets from the nozzle orifice. Supply of the decompressing component and the decompressed state holding component of this microdot drive pulse causes a central portion of the meniscus of ink (that is, a free surface of ink, which is exposed to the nozzle orifice) to protrude in such a manner as to have a columnar shape. Further, supply of the discharging component causes the recording head to eject the columnar portion of the ink as an ink droplet.
When the recording head is driven by such a microdot drive pulse, an ink droplet is divided into a main ink droplet, which is separated from an end portion of an ink column and jetted, and a satellite ink droplet that is jetted in such a way as to accompany the main ink droplet. The jetting speed of this satellite ink droplet is lower than that of the main ink droplet. Moreover, the amount of ink of the satellite ink droplet is less than that of ink of the main ink droplet. For example, when the jetting speed of the main ink droplet is about 7 m/s, that of the satellite ink droplet is approximately 4 m/s. Furthermore, the amount of ink of the satellite ink droplet is two thirds of that of the main ink droplet.
In recent years, it is demanded a recording apparatus capable of recording image with further improved quality. It is necessary for meeting this demand to more reduce an ink amount of each ink droplet. However, when an ink droplet is ejected in response to a related microdot drive pulse, the amount of ink of the satellite ink droplet is extremely small. For instance, when about 1.5 pL of ink of an ink droplet is ejected, the amount of ink of the satellite ink droplet is about 0.5 pL.
Thus, the satellite ink droplet is largely affected by the viscous resistance of air. Consequently, the jetting speed of the satellite ink droplet is largely reduced until the satellite ink droplet impacts on recording paper. On the other hand, generally, the amount of ink of the main ink droplet is more than that of ink of the satellite ink droplet. Thus, the degree of reduction in the speed of the main ink droplet is lower than that of reduction in the speed of the satellite ink droplet. Consequently, when the droplet impacts on the recording paper, the difference in the jetting speed between the main ink droplet and the satellite ink droplet increases still more. Further, the ejection of the ink droplets is performed by simultaneously moving the recording head. This raises the problems that the impact positions of the main ink droplet and the satellite ink droplet deviate from each other owing to the difference in the jetting speeds, and that the image quality is degraded contrary to the demand.
Moreover, there is a probability that the satellite ink droplets cannot reach the recording paper due to the air resistance. In such a case, the satellite ink droplets float as ink mists. When such ink mists adhere to a casing and a nozzle plate of the recording head, the deflection of flight path of the ink droplet and the contamination of the inside of the apparatus are caused, so that the reliability of the apparatus is degraded.
Accordingly, a first object of the invention is to provide a driving method for an ink jet recording head enabled to increase the jetting speed of an ink droplet even when an ink amount of the ink droplet is extremely small, and to provide an ink jet recording apparatus incorporating such a recording head.
A second object of the invention is to make the impact positions of a main ink droplet and a satellite ink droplet coincide with each other, while preventing the satellite ink droplets from becoming ink mists, thereby to improve the image quality.
In order to achieve the above objects, according to the present invention, there is provided a method of driving an ink jet recording head provided with a pressure chamber communicated with a nozzle orifice from which a main ink droplet and a satellite ink droplet accompanied with the main ink droplet are ejected to form an ink dot on a recording medium, comprising the steps of:
generating pressure fluctuation in the pressure chamber;
ejecting ink contained therein from the nozzle orifice as the main ink droplet having a first volume, due to the pressure fluctuation; and
ejecting ink contained therein from the nozzle orifice as the satellite ink droplet having a second volume which is larger than the first volume, due to the pressure fluctuation.
In this configuration, the satellite ink droplet becomes more resistible to the influence of the viscous resistance of air, as compared with the main ink droplet. Moreover, the rate of reduction in the ratio of the speed to the flight distance of the satellite ink droplet becomes smaller than that corresponding to the main ink droplet. This enables reduction in the difference in the speed at the impact position between the main ink droplet and the satellite ink droplet. Thus, the deviation of the impact position of the satellite ink droplet from that of the main ink droplet is decreased. Consequently, even when an extremely small ink droplet is ejected, the impact position of the main ink droplet is made to coincide with that of the satellite ink droplet, so that the image quality of the recorded image is improved. Furthermore, the amount of ink of the satellite ink droplet, the jetting speed of which is liable to be low, is more than that of the main ink droplet. Thus, the ink droplets are enabled to reliably land onto the recording medium. Consequently, the ink droplets are prevented from becoming ink mists.
There may be accompanied plural satellite ink droplets. In this case, the amount (second volume) of at last one of these satellite ink droplets is more than the amount of ink of a main ink droplet.
Preferably, the pressure fluctuation generating step includes the steps of:
decompressing the pressure chamber such that a central portion of a meniscus of ink in the nozzle orifice is locally pulled toward the pressure chamber; and
compressing the pressure chamber when the pulled meniscus reactionally moves in a direction in which the ink droplets are ejected.
Here, it is preferable that the decompressing step includes the steps of:
decompressing the pressure chamber with a first decompressing force so as to pull the meniscus toward the pressure chamber while keeping a shape of the meniscus in a stationary state thereof;
decompressing the pressure chamber with a second decompressing force which is greater than the first decompressing force, so as to locally pull the central portion of the meniscus; and
holding the decompressed state of the pressure chamber until the pulled meniscus reactionally moves in the direction in which the ink droplets are ejected.
Here, the expression xe2x80x9cstationary statexe2x80x9d designates a state in which extremely small pressure fluctuation occurs in the pressure chamber, and in which the meniscus is placed in the vicinity of a nozzle formation face, that is, a state in which ink is filled in the nozzle orifice. Further, the expression xe2x80x9ckeeping a shape of meniscusxe2x80x9d means that a slight change in curvature is permitted.
In this configuration, since the meniscus is pulled toward the pressure chamber while keeping the shape of the meniscus in the stationary state thereof, the inertance at the nozzle orifice side is lowered. Thus, the response of the meniscus to the pressure fluctuation of ink in the pressure chamber is enhanced so that the central portion of the meniscus can be locally pulled by rapidly decompressing the inside of the pressure chamber. Moreover, since the pressure chamber is compressed in synchronization with timing with which the pulled central portion of the meniscus moves in the ejecting direction as a reaction, the pressure of ink from the pressure chamber is applied to the central portion of the meniscus during the reaction, so that the central portion of the meniscus, which becomes an ink droplet, is strongly pushed out. Consequently, the jetting speed of the ink droplet is increased. Therefore, the ink droplet obtains a sufficient jetting speed even when the amount of ink of the droplet is extremely small. Thus, the ink droplet is impacted onto a desired place. Consequently, the image quality of the recorded image is further improved.
According to the present invention, there is also provided a method of driving an ink jet recording head provided with a pressure chamber communicated with a nozzle orifice from which a main ink droplet and a satellite ink droplet accompanied with the main ink droplet are ejected to form an ink dot on a recording medium, comprising the steps of:
generating pressure fluctuation in the pressure chamber;
ejecting ink contained therein from the nozzle orifice as the main ink droplet with a first speed, due to the pressure fluctuation; and
ejecting ink contained therein from the nozzle orifice as the satellite ink droplet with a second speed which is faster than the first speed, due to the pressure fluctuation.
In this configuration, the satellite ink droplet ejected after the ejection of the main ink droplet is jetted at a speed that is higher than the speed, at which the main ink droplet ejected before the ejection of the satellite ink droplet is jetted. This can bring the impact position of the main ink droplet on the recording paper close to that of the satellite ink droplet. Consequently, even when an extremely small ink droplet is ejected, the impact positions of a main ink droplet and a satellite ink droplet can be made to coincide with each other thereby to improve the image quality. Furthermore, the jetting speed of the satellite ink droplet, the amount of ink of which is liable to be small, is higher than that of the main ink droplet. Thus, the ink droplets are enabled to reliably land onto the recording paper. Consequently, the ink droplets are prevented from becoming ink mists.
Preferably, the pressure fluctuation generating step includes the steps of:
compressing the pressure chamber with a first compressing force at a timing when the main ink droplet is separated from a meniscus of ink in the nozzle orifice; and
compressing the pressure chamber with a second compressing force which is greater than the first compressing force, at a timing when the satellite ink droplet is separated from the meniscus.
Here, it is preferable that the decompressing component includes:
a first decompressing component, which decompress the pressure chamber with a first decompressing force so as to pull the meniscus toward the pressure chamber while keeping a shape of the meniscus in a stationary state thereof; and
a second decompressing component, which decompress the pressure chamber with a second decompressing force which is greater than the first decompressing force, so as to locally pull the central portion of the meniscus.
Further, it is preferable that the nozzle orifice has a first part in which a diameter thereof is constant, and a second part in which the diameter is enlarged toward the pressure chamber. The central portion of meniscus pulled by the second decompressing component is placed in the second part of the nozzle orifice.
In this configuration, the inertance at the nozzle orifice side is further lowered. Therefore, the response of the meniscus to the pressure fluctuation of ink contained in the pressure chamber is further enhanced. Consequently, the jetting speed of the ink droplet is easily increased.
Preferably, a termination end of the second decompressing component and an initial end of the compressing component is connected by a first holding component which maintains a potential of the termination end of the second decompressing component.
In this configuration, the meniscus performs free vibration over a time during which the first holding component is supplied. Furthermore, since the first holding component provides timing with which the supply of the compressing component is started. Thus, the timing, with which the central portion of the meniscus is pushed out, is optimized according to the setting of the time period during which the first holding component is supplied.
Preferably, the drive signal includes a damping component which follows the compressing component and compresses the pressure chamber so as to prevent the meniscus after the ink droplet ejection from reactionally moving toward the pressure chamber.
In this configuration, the vibration of the meniscus due to the ink ejection is settled in a short time. Thus, a time interval required for enabling the next ejection of an ink droplet is shortened. Thus, a printing period is shortened. Consequently, the printing speed is increased.
Here, it is preferable that a termination end of the compressing component and an initial end of the damping component is connected by a second holding component which maintains a potential of the termination end of the compressing component.
In this configuration, the timing, with which the supply of the damping component is commenced, is determined according to the time period during which the second holding component is supplied. Thus, the timing, with which the pressure chamber is compressed, is optimized. Consequently, the damping is effectively performed.
According to the present invention, there is also provided an ink jet recording apparatus, comprising:
a recording head, provided with a pressure chamber communicated with a nozzle orifice;
a pressure generating element, which generates pressure fluctuation in ink contained in the pressure chamber; and
a drive signal generator, which generates a drive signal for driving the pressure generating element such that a main ink droplet and a satellite ink droplet accompanied with the main ink droplet are ejected to form an ink dot on a recording medium, the main ink droplet having a first volume, and the satellite ink droplet having a second volume which is larger than the first volume.
Preferably, the drive signal includes:
a decompressing component, which decompresses the pressure chamber such that a central portion of a meniscus of ink in the nozzle orifice is locally pulled toward the pressure chamber; and
a compressing component, which compresses the pressure chamber when the pulled meniscus reactionally moves in a direction in which the ink droplets are ejected.
According to the present invention, there is also provided an ink jet recording apparatus, comprising:
a recording head, provided with a pressure chamber communicated with a nozzle orifice;
a pressure generating element, which generates pressure fluctuation in ink contained in the pressure chamber; and
a drive signal generator, which generates a drive signal for driving the pressure generating element such that a main ink droplet and a satellite ink droplet accompanied with the main ink droplet are ejected to form an ink dot on a recording medium, the main ink droplet ejected with a first speed, and the satellite ink droplet ejected a second speed which is faster than the first speed.
Preferably, the drive signal includes:
a first compressing component, which compresses the pressure chamber with a first compressing force at a timing when the main ink droplet is separated from a meniscus of ink in the nozzle orifice; and
a second compressing component, which compresses the pressure chamber with a second compressing force which is greater than the first compressing force, at a timing when the satellite ink droplet is separated from the meniscus.
Here, it is preferable that a potential gradient of the second compressing component is steeper than a potential gradient of the first compressing component.
Preferably, the pressure generating element is an electromechanical transducer.
Here, it is preferable that the electromechanical transducer is a piezoelectric vibrator.
According to the present invention, there is also provided a method of driving an ink jet recording head provided with a pressure chamber communicated with a nozzle orifice from which a main ink droplet and a satellite ink droplet accompanied with the main ink droplet are ejected to form an ink dot on a recording medium, comprising the steps of:
decompressing the pressure chamber while lowering an inertance of ink situated closer to the pressure chamber than an inertance of ink situated closer to the nozzle orifice; and
compressing the pressure chamber to eject the main ink droplet and the satellite ink droplet.
Preferably, a shape of the nozzle orifice is determined so as to realize the inertance lowering.
Preferably, an intensity and a duration time period of a decompressing force generated in the decompressing step is determined so as to realize the inertance lowering.